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Changes of Reporting Rates in the Southern California Earthquake Catalog, Introduced by a New Definition of M_L

Tormann, Thessa and Wiemer, Stefan and Hauksson, Egill (2010) Changes of Reporting Rates in the Southern California Earthquake Catalog, Introduced by a New Definition of M_L. Bulletin of the Seismological Society of America, 100 (4). pp. 1733-1742. ISSN 0037-1106. doi:10.1785/0120090124.

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Starting January 2008, local magnitudes M_L for southern California are determined by a new calibration that provides various improvements for determining M_L for small earthquakes. Magnitudes for the previous years are being recalculated and the catalog continuously updated, with the first year of overlapping data now being available. Recalibrating a magnitude scale can cause a break in homogeneity of reporting and often produces artifacts in the catalog statistics that can influence a wide range of seismicity studies. To search for such a break, we compare the old M_L and the new M_L catalogs for 2007. We find (1) the two magnitude values differ for 96% of the M_L events, and hand-determined magnitudes are also revised; (2) the magnitude differences are irregular from magnitude increases of up to 1.5 units to reductions by as much as 2.3 units, with an average change of -0.13 units; (3) the number of events above M 1.8 decreases by 32% for the new magnitude scale; (4) the completeness magnitude apparently drops by 0.3 units from 1.6 to 1.3; (5) the b-value reduces by approximately 0.2 units, dropping from 1.16 to 0.95; (6) the new magnitude calibration produces a more stable b-value estimate and can therefore be regarded as the better scaling. We document selected examples of how the change in magnitude calibration may affect seismicity- and hazard-related analyses that are based on the Southern California Seismic Network (SCSN) catalog. Especially the change of the b-value from ~1.1 to ~0.9 has potentially major implications for hazard related applications.

Item Type:Article
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URLURL TypeDescription DOIArticle
Hauksson, Egill0000-0002-6834-5051
Additional Information:© 2010 Seismological Society of America. We thank Jochen Woessner, Annemarie Christophersen, Kate Hutton, Max Wyss, Duncan Agnew, one anonymous reviewer, and the editorial board for valuable feedback that helped improve the manuscript. This research was supported by the Southern California Earthquake Center. SCEC is funded by the National Science Foundation Cooperative Agreement EAR-0106924 and USGS Cooperative Agreement 02HQAG0008. The SCEC contribution number for this article is 1408.
Group:Seismological Laboratory
Funding AgencyGrant Number
Southern California Earthquake Center (SCEC)UNSPECIFIED
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Other Numbering System NameOther Numbering System ID
Southern California Earthquake Center 1408
Issue or Number:4
Record Number:CaltechAUTHORS:20100817-133638311
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:19474
Deposited By: Tony Diaz
Deposited On:17 Aug 2010 20:45
Last Modified:08 Nov 2021 23:52

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